[Theme: Church] opacity to content background

im totally new to this whole wordpress thing and themes,
but i am using the “church” theme and i believe it features the omega child theme?

everybody is talkin bout working with child themes so thats probably the way to go, but i have no clue how to do things.
all i want to do is add a bit of opacity to the content (white) area of my posts/articles etc. So that the background image shines through but not for a 100%, just so the text is readable.

please, can anyone give me some advice on how to do this?

my website is http://www.momicz.com